1. Field of the Invention
The present invention relates to a lens apparatus provided with a mechanism to move a lens in an optical axis direction and interchangeable with respect to or integrated with an image pickup apparatus.
2. Description of the Related Art
Lens apparatuses used for image pickup apparatuses such as single-lens reflex cameras are provided with a mechanism to move a lens (and a lens holding member holding the lens) in an optical axis direction for variation of magnification or focusing. Such a mechanism includes a clearance gap for enabling smooth movement of the lens. However, change of a direction in which the clearance gap narrows with change of posture of the image pickup apparatus including the lens apparatus may cause change of optical performance of the lens apparatus. Therefore, a countermeasure is required which prevents the direction in which the clearance gap narrows from changing with the change of the posture of the image pickup apparatus.
Japanese Patent Laid-Open Nos. 2002-267919 and 2007-101634 disclose a mechanism that always biases one of two members with a spring so as to press it against the other member. Specifically, the mechanism disclosed in Japanese Patent Laid-Open No. 2002-267919 includes a spring provided between two lens holding members movable in an optical axis direction such that the spring extends in a direction oblique to the optical axis direction and a biasing force generated by the spring presses cam followers provided in the two lens holding members against one side cam surfaces of two cam groove portions formed in a cam ring. On the other hand, the mechanism disclosed in Japanese Patent Laid-Open No. 2007-101634 includes a coil spring wound about an axis parallel to an optical axis direction and twisted such that a rotational biasing force generated by the twisted coil spring in a plane orthogonal to the optical axis direction presses a nut provided in a lens holding member against a lead screw formed on an output shaft of a motor.
The mechanism disclosed in Japanese Patent Laid-Open No. 2002-267919 provides a sufficient pressing function when the two lens holding members are only movable in the optical axis direction without rotating about an optical axis. However, when one of the two lens holding member is rotatable with respect to the other lens holding member about the optical axis, rotation of the one lens holding member changes a generation direction of the biasing force of the spring extending in the direction oblique to the optical axis direction. Moreover, as disclosed in Japanese Patent Laid-Open No. 2007-101634, even though the spring is used which generates the rotational biasing force by being twisted about the optical axis, the rotation of the one lens holding member changes a twisted amount of the spring, which results in change of strength of the rotational biasing force. Such change of the direction and strength of the biasing force may provide insufficient pressing and thereby may change optical performance with posture change.